The present invention relates to devices for detecting, monitoring and/or measuring low-level concentrations of certain materials within a complex matrix stream or aggregation of flowable materials. In one of several, more particular aspects, the present invention relates to such devices which employ permeation membrane or diffusion membrane tubing in some fashion to collect and isolate these certain materials from within the stream or aggregation.
Tubular membranes have been suggested for use in this capacity in a number of analytical devices, and are particularly of interest for use in separating out those components of a complex stream or aggregation which might adversely affect a gas or liquid chromatograph or other analytical device if one attempted to analyze the stream or aggregation directly.
Because of the harsh sampling environment presented by many process streams and because of the fragility of the .suitable tubular membranes, however, the previously known devices employing such tubular membranes have very largely been designed to operate on but a portion of the larger stream or aggregation.
The present inventors have previously developed and described a device in the referenced U.S. patent application Ser. No. 07/843,687 which is amenable to sampling from difficult process streams or aggregations by direct insertion. This feature represents a significant improvement over the previously-known devices. One formerly unresolved problem that the inventors have noted with a direct insertion device, though, relates to the influence of temperature on the membrane's permeation/transfer characteristics and accuracy.
In those devices wherein a small sample stream or aggregation is operated upon, it has been possible in the past to monitor and control the temperature of the sample stream or aggregation and to thus monitor and control, indirectly, the temperature at the tubular membrane/fluid interface. Obviously this is not going to be a very practical approach in the context of directly inserting a sample probe into most process streams of interest. And even in the context of a small sample stream (or slipstream) or aggregation, thermal conditioning of the entire sample stream or aggregation can be energy-intensive and troublesome, for example in causing degassing or precipitation of other soluble species in the stream or aggregation.
One device is reported in the literature for effecting a separation of a high-boiling organic solute (e.g., phenol, 4-hydroxydecanoic acid lactone) from a dilute aqueous solution of the solute via a heated tubular membrane instead of by thermal conditioning of the aqueous solution, see Haggin, "Heated Membranes Give Improved Separation", Chemical and Engineering News, Apr. 27, 1992, page 41. In this device, the membrane is internally reinforced with a woven wire mesh that can be heated and which enhances the desired separation, whereas heating the aqueous solution was found in the case of the phenol/water system to increase organic flux density but to reduce separation of the phenol from water.